Abstract <p>The optimal time for applying 1 M nitric acid solution to carbonized sapropelic coal that is intended for use in supercapacitor electrodes is determined: modification for 10 min at 100°C is required. This approach increases the specific capacitance of the resulting electrode material by 20%, on average, with different potential scanning rates. Sorption measurements establish that modification of the carbonizate by nitric acid increases the mean mesopore size. In turn, in the production of Co₃O₄/C nanocomposite by an absorption method, with subsequent thermal decomposition of the precursor—cobalt(II) nitrate—at the surface and within the pores of the carbonizate, the optimal time for modification of the carbon matrix by nitric acid is 15 min at 100°C. The specific capacitance of the resulting nanocomposite is ~359 F/g. That is 1.25 times the value for the initial electrode material.</p>